1. Field of the Invention
The present invention relates for example to a punch for pressing a powder such as a rare-earth alloy powder into a compact, for manufacture of a magnet used in a voice coil motor, a powder pressing apparatus and a powder pressing method using the above punch, a compact formed by such a pressing method as above, a sintered body and a voice coil motor using the sintered body.
2. Description of the Related Art
Referring to FIG. 14 and FIG. 15, description will cover a conventional method for pressing the rare-earth alloy powder into a compact 8 (see FIG. 16). The compact 8 is sintered and then used as a magnet for the voice coil motor and so on.
In order to manufacture the compact 8, a tolling 1 for pressing operation as shown in FIG. 14 is used. The tolling 1 includes a die 3 having a through hole 2, a lower punch 4 to be inserted into the through hole 2 in advance, and an upper punch 5 to be inserted into the through hole 2. The lower punch 4 is provided with an upper surface having a center portion formed with a generally arcuate projection 4a and two edge portions each formed with a flange-like projections 4b. The upper punch 5 has a lower surface formed with a recess 5a. Each of the lower punch 4 and the upper punch 5 is made of a hard alloy such as cemented carbide, and for protection from chipping and cracking, each tip of the projection 4b and an edge portion 5b is chamfered by 0.8 mm.
When pressing, first, the lower punch 4 is lowered to form a cavity 6 in the through hole 2, and the cavity 6 is fed with a rare-earth alloy powder 7.
Then, the rare-earth alloy powder 7 in the cavity 6 is pressed between the lower punch 4 and the upper punch 5 while being oriented by a magnetic field. The pressing operation to the rare-earth alloy powder 7 is continued until the two edge portions 5b of the upper punch 5 are about to contact the corresponding projections 4b of the lower punch 4 (until a gap between the punches becomes about 1 mm for example) as shown in FIG. 15 in order to form a shape as close as of a final product.
As a result, a compact 8 as shown in FIG. 16 is obtained.
The compact 8 is formed to have a generally arcuate section, including an upper surface 8a formed by the recess 5a of the upper punch 5, a lower surface 8b formed by the projection 4a of the lower punch 4, slopes 8c formed by the projection 4b of the lower punch 4, and end surfaces 8d formed by a wall of the through hole 2.
As shown in FIG. 16, the compact 8 has a problem that cracks A develop along border portions 8e between the upper surface 8a and the slope 8c.
Causes of the crack A will be described.
As shown in FIG. 14, when the rare-earth alloy powder 7 is fed into the cavity 6, layers of a marking material B having a color different from that of the rare-earth alloy powder 7 were inserted at a predetermined interval, and then the pressing operation was made. Then, as shown in FIG. 15, the gap between layers of the marking material B was found to be very narrow between the projection 4b of the lower punch 4 and the edge portion 5b of the upper punch 5. This indicates that the rare-earth alloy powder 7 has a much higher density between the projection 4b of the lower punch 4 and the corresponding edge portion 5b of the upper punch 5 than in other portions. This is presumably that when being pressed, the rare-earth alloy powder having a poor flowability was sandwiched between a tip of the projection 4b of the lower punch 4 and a tip of the edge portion 5b of the upper punch 5, became unable to move, and was compressed into such a high density at the border portion 8e. Therefore, pressure exerted to the compact 8 is relieved when the compact 8 is taken out of the through hole 2 of the die 3, allowing a highly compressed portion such as the border portion 8e to expand more significantly, often developing cracks or fractures. A similar problem occurs in a sintering process.
If the rare-earth alloy powder 7 is pressed in a strong magnetic field not smaller than 0.5 MA/m oriented in a direction indicated by an arrow c (longitudinally of the projection 4b) shown in FIG. 16, grains of the magnetized rare-earth alloy powder 7 repel against each other, making the powder density higher in a region closer to a perimeter of the cavity 6 than in a center portion of the cavity 6, increasing further the density near the projection 4b. Further, if the rare-earth alloy powder 7 fed into the cavity 6 is wiped flush at an upper edge of the cavity 6 by a lower edge of feeder box (not shown), region to be pressed by the projection 4b is fed with an excess mount of the rare-earth alloy powder 7 than needed. Since the rare-earth alloy powder 7 does not have enough flowability, the density in this region after the pressing becomes higher than in other regions. Therefore, in these cases, the compact 8 becomes more apt to develop the cracks or fractures in the border portion 8e when taken out of the cavity 6.